1. Field of the Invention
The present invention relates to an aircraft propulsion system comprising a bypass turbojet engine having at least a compressor, a combustion chamber, a turbine, and a fan mounted upstream from the compressor and driven in rotation by the turbine, the engine being surrounded by a nacelle that, downstream from the fan, defines an annular flow section for a secondary stream of air around the casings of the compressor, of the combustion chamber, and of the turbine, this secondary stream providing the major fraction of the thrust.
The propulsion system is attached to a structural element of an aircraft, such as a wing of the aircraft, by means enabling it to transmit to said structural element the forces that are generated by the engine during its various operating stages.
2. Description Of The Related Art
The turbojet engines of modern civilian airplanes are characterized by a large bypass ratio, i.e. a ratio of secondary flow rate divided by primary flow rate that is greater than 5 and that may be as much as 9 or 10, in particular for the purpose of reducing the noise nuisance and reducing the fuel consumption of such engines. This gives rise to a reduction in the transverse dimensions of the turbojet engine between the fan and the turbine (“wasp waist” effect) that reduces the bending strength of the body.
Bending deformations of the body of the turbojet themselves give rise to deformations of the casing around the rotor, causing the casing to be ovalized, thereby reducing the clearances between the casing and rotor at certain locations and increasing the clearances at other locations (“casing distortion” effect).
This effect reduces the performance of the turbojet, in particular because it makes it necessary to provide larger clearances at the tips of the blades.
The means for attaching the propulsion system generally comprise a strong and massive part commonly referred to as a pylon or strut, together with suspension means connecting the engine to the pylon. Such suspension means are conventionally fastened firstly to one or more casings of the engine, e.g. upstream on an intermediate casing and downstream on an exhaust casing, and secondly on a bottom portion of the pylon that extends into the secondary flow section and that is generally surrounded by walls for guiding the secondary stream, thus forming what is sometimes called a “12o'clock divider”.
Those known fastener means present several drawbacks.
Attaching the propulsion system to the airplane pylon via suspension means fastened to casings gives rise to pinching of the casings and enhances effects of distortion and misalignment of the various rotary elements of the engine, thereby leading to losses of efficiency and giving rise to an increase in fuel consumption.
Furthermore, take-up of the torsion torque induced by the rotation of the rotary elements of the engine via the downstream suspension requires attachment means to be used that are large and requires a 12 o'clock divider to be that much wider when a pylon penetrates at least in part into the secondary flow section.
For a given bypass ratio, that therefore requires the radial dimensions of the nacelle to be increased, thereby increasing drag and making it difficult to incorporate turbojets having a large bypass ratio.